What solar term did the cold weather start from?

Nine cold days begin with the winter solstice solar terms.

Winter solstice (alias: 1999, Winter Festival, Dragon Festival, Asian Year, English name: Winter solstice) is the 22nd solar term in the "24 solar terms", the longitude of the sun reaches 270 degrees, and the date falls on the 22nd or 23rd of February in the Gregorian calendar every year, which is a traditional festival of the Chinese nation. The custom of winter solstice originated in Han Dynasty and flourished in Tang and Song Dynasties.

The solstice in winter is the extreme point of direct sunlight spreading to the south. From winter to the sun, the sun shines directly on the tropic of Capricorn, and the sun is most inclined to the northern hemisphere, and the elevation angle of the sun is the smallest. In all parts of the northern hemisphere, the days are the shortest and the nights are the longest. The solstice in winter is also a turning point for the sun to return directly to the north. After this day, there will be a "turn back", and the direct point of the sun will move northward from the tropic of Capricorn (23 26' s), and the daytime in the northern hemisphere will increase day by day.

There is a folk proverb that "the solstice in winter is as big as a year", but during the solstice in winter, different places have different customs. Most northerners eat jiaozi, while most southerners eat jiaozi.

The origin of solar terms:

China's astrological culture has a long history and is profound. The twenty-four solar terms were originally formulated on the basis of changing stars. In different seasons and at different times, the Big Dipper will point in different directions, that is, "bucket handle points to the East, and the world is spring; Guide to barrel handle, it's summer in the world; Bucket handle refers to the west, and the world is autumn; Bucket handle refers to the north, and it is winter in the world. " . "Twenty-four solar terms" is the product of ancient farming civilization, which is related to the time of gossip and has a long history.

During the period of Emperor Wu of the Han Dynasty, the "Twenty-four solar terms" were absorbed into the taichu calendar as a supplement to the calendar to guide agriculture in the northern part of the Central Plains, and the "Twenty-four solar terms" were measured in the Yellow River basin by the method of solar shadow measurement. The current "twenty-four solar terms" are determined according to the position of the sun on the ecliptic, that is, on a "ecliptic" with a circumference of 360 degrees (the apparent path of the sun on the celestial sphere for one year), the spring is divided into 0 degrees, and every time the sun runs on the ecliptic, 15 degrees is a "solar term".